In a process for producing a semiconductor device such as a microprocessor, a memory or CCD, or a flat panel display device such as a TFT liquid crystal, formation of a pattern or formation of a thin film is carried out in a submicron size on the surface of a substrate of e.g. silicon (Si), silicon oxide (SiO2) or glass. And, in such production, it is very important to reduce the very small amount of contaminants on the substrate surface in each step for the production.
Among contaminants, particularly metal contaminants or particle contaminants deteriorate the yield or the electrical characteristics of the device. Accordingly, it is necessary to reduce contaminants as far as possible within the step wherein the contaminants are formed and then transfer the substrate to the next step. To remove the contaminants, it is common to clean the substrate surface with a cleaning agent.
In recent years, in the production of devices, further improvement in the production efficiency, such as improvement in the number of substrates treated per unit time, is required. In the production of a substrate having a device which tends to be increasingly microsized and highly integrated, a cleaning method is desired which is capable of highly cleaning the substrate surface quickly and which is excellent in the ability to remove both metal contaminants and particle contaminants on the metal surface.
It is commonly known that for the removal of particle contaminants, cleaning with an alkaline solution is effective. In cleaning the surface of a Si substrate or a SiO2 substrate for a semiconductor device, or a glass substrate for a display device, an aqueous alkaline solution such as an aqueous ammonia solution, an aqueous potassium hydroxide solution or an aqueous tetramethylammonium hydroxide solution, is employed. Further, cleaning (so-called “SC-1 cleaning” or “APM cleaning”) with a cleaning agent (so-called “SC-1 cleaning agent” or “APM cleaning agent”) comprising ammonia, hydrogen peroxide and water, is also widely employed.
On the other hand, it is known that for the removal of metal contaminants, cleaning with an aqueous acidic solution is effective. In cleaning the surface of a Si substrate or a SiO2 substrate for a semiconductor device, or a glass substrate surface for a display device, one having an acid such as hydrochloric acid, nitric acid, sulfuric acid or hydrofluoric acid diluted with water, is employed as a cleaning agent. Further, cleaning (so-called “SC-2 cleaning” or “HPM cleaning”) with a cleaning agent (so-called “SC-2 cleaning agent” or “HPM cleaning agent”) comprising hydrochloric acid, hydrogen peroxide and water, is also widely employed.
In order to remove both particle contaminants and metal contaminants on the substrate surface, cleaning is carried out by a combination of such cleaning methods in a plurality of steps. As a method for cleaning contaminants on the surface of a substrate for a semiconductor device, a so-called RCA cleaning (Kern and Puotinen: RCA Review, pp. 187-206, June (1970), etc.) is widely used in which particle contaminants are removed by SC-1 cleaning and then metal contaminants are removed by SC-2 cleaning. Further, in order to remove metal contaminants firmly chemically bonded to Si or SiO2 on the substrate surface, or a very small amount of metal contaminants taken into the interior of the surface layer of the substrate, it is effective to etch the surface layer of the substrate with a dilute hydrofluoric acid aqueous solution having a hydrofluoric acid content of from about 0.25 to 1 wt % (hereinafter sometimes referred to simply as “dilute hydrofluoric acid”). Particularly, aluminum (Al) and copper (Cu) are likely to react with Si or SiO2 on the substrate surface to form a firm chemical bond and are likely to be taken into the vicinity of the surface layer of the substrate, and in recent years, it has been found that in order to clean such a substrate surface, it is most effective to employ a method of removing them by etching the surface layer of the substrate (Morinaga et al., The Electrochemical Society Proceeding Series PV99-36, pp. 585-592, Pennington, N.J. (2000) etc.). Thus, after SC-1 cleaning or after SC-2 cleaning, or between SC-1 cleaning and SC-2 cleaning, cleaning with dilute hydrofluoric acid is carried out, so that a highly clean surface can be obtained by carrying out cleaning in a total of three steps. In such a two step or three step cleaning method, it usually takes from 1 to 15 minutes per one cleaning step, and it requires a total of a few tens minutes.
Further, as apparatus for cleaning the surface of a substrate for a device, there are two types of typical ones. One type is a batch system cleaning apparatus wherein a plurality of substrates accommodated in a cassette is immersed and cleaned in a cleaning tank which usually contains a cleaning agent. The other type is a sheet system cleaning apparatus wherein a single sheet of a substrate is mounted on a holder, and usually, while rotating the substrate (for example, if the substrate is a disk, it is rotated in a circumferential direction), a cleaning agent is sprayed to the substrate surface.
The batch type cleaning apparatus is capable of treating a large number of substrates per unit time, but has problems such that the apparatus is large, so-called re-deposition of contaminants (crosscontamination) among substrates is likely to result such that contaminants detached from a device-formed surface of a substrate or from its rear side will deposit on another device-formed surface, and even when it is desired to clean only one sheet, a large amount of the cleaning agent will be required.
On the other hand, the sheet system cleaning apparatus is free from crosscontamination and the apparatus is small, but it has a problem that the number of substrates which can be treated per unit time, is small, since only one sheet is cleaned each time.
As mentioned above, in the conventional method for cleaning the substrate surface, the substrate surface has been highly cleaned by a cleaning method having a plurality of steps including SC-1 cleaning, SC-2 cleaning and further dilute hydrofluoric acid cleaning, etc. Accordingly, there have been problems that the cleaning time is long, and the production efficiency is poor. Particularly by a sheet system cleaning apparatus wherein only one substrate sheet is cleaned each time, the number of substrates which can treated per unit time per one apparatus is small, whereby there have been problems that many cleaning apparatus are required, and such is disadvantageous from the viewpoint of costs.
Further, in the dilute hydrofluoric acid cleaning, various problems have been noticed in recent years, for example, a problem such that during usual cleaning treatment for from 1 to 5 minutes by means of an aqueous hydrofluoric acid solution having a concentration of about 0.5 wt %, the substrate surface (a SiO2 film or the like) will be etched by at least 10 Å, whereby the dimensional precision of the device on the substrate surface will be impaired. Such a problem is expected to be rapidly noticed in future as the device will be increasingly microsized, and the gate oxide film will be thinner.
Further, there are also problems such that particles are likely to deposit and contaminate the hydrophobic Si surface exposed by etching (usually, particles are more likely to deposit on a hydrophobic surface), and further, a stain-like soiling which forms when a waterdrop remaining on a hydrophobic surface dries up, i.e. a so-called watermark, is likely to form.
In order to control such etching of SiO2, a cleaning method is also proposed wherein, although the cleaning time is the same as before, the hydrofluoric acid concentration is extremely reduced, and for example, the substrate surface is cleaned with an extremely dilute hydrochloric acid aqueous solution having a very small amount at a level of e.g. 10 wt ppm of hydrofluoric acid added to pure water (JP-A-3-190130, etc.). However, by such a cleaning method, since the hydrofluoric acid concentration is extremely low, it requires about 5 minutes for only cleaning with this hydrofluoric acid aqueous solution, whereby the production efficiency is still low. In addition, in a case where this method is applied to a sheet cleaning apparatus, a large amount of the cleaning agent will be required, and accordingly a large amount of an acid waste liquid will be formed, and its disposal will be problematic.